1. Field of the Disclosure
The present subject matter is generally directed to wireline tools that are used for well logging operations, and more specifically to a noise isolation tool that may be used during a noise logging operation.
2. Description of the Related Art
Since the earliest wells were constructed during the 19th Century, controlling what would otherwise be an uncontrolled flow or migration of hydrocarbons to the surface has been an ongoing challenge to the oil and gas industry. In modern oil and gas wells, such uncontrolled flow or migration can often be attributed to inadequate or improper isolation of the various producing zones through which a wellbore has been drilled. For example, in some cases, poor zonal isolation can lead to the uncontrolled flow of gas and/or liquid to the surface by way of the annulus between the production casing (or if present, an intermediate casing) and the surface casing. Such uncontrolled flow through the surface casing annulus is sometimes referred to as internal migration, or surface casing vent flow (SCVF). In other cases, a flow of gas may be detectable at the surface outside of the outermost casing string, i.e., the surface casing, or if present, the conductor casing of the well. Such flow outside of the outermost casing is sometimes referred to as external migration, seepage, or more simply, gas migration (GM). Gas migration can be a serious occurrence if there is there is a possibility of fire, a public safety hazard, and/or environmental damage, such as groundwater contamination and the like.
Many factors can contribute to the uncontrolled flow of gas up a wellbore. For example, conditions directly related to the initial drilling of the wellbore, the casing and/or cementing design, the completion techniques utilized, and/or the age of a wellbore can all influence the eventual uncontrolled flow of gas in a well. Moreover, poor casing quality, improper displacement of drilling mud during drilling activities, poor cement slurry design and/or pumping practices, damage to the primary cement job after drilling, and the recovery methods used on the a well, such as steam assisted gravity drainage (SAGD) and/or other steam flood regimes, can also contribute to the creation of gas flow paths in the wellbore. Some of these factors are briefly discussed below.
In some wells, lower quality casing materials, or casing strings that are improperly placed, i.e. over torque collars and the like, can create leakage conditions which can thereby locally charge formations to create the surface casing vent flow and/or gas migration issues described above. Furthermore, improper circulating and wellbore conditioning practices can sometimes leave a mud column, or a mud cake, between the casing and the formation, which may limit the ability of the primary cement column to create an effective bond that would normally be required to inhibit and stop undesirable gas flow.
Poor cement slurry designs and improper or inadequate cement placement during the cementing operation can sometimes cause channeling effects in the cement/mud column, thereby creating a flow path in the wellbore. Improper squeeze cementing practices can contribute to gas-infused cement columns, which may also leave a flow path for gas. Additionally, damage to the cement column can sometimes occur after the primary cementing operation has been completed on a well. For example, pressures that are exerted on the wellbore during routine completion operations, such as pressure testing and/or hydraulic fracturing, as well as various thermal operational modes, such as steam assisted gravity drainage and/or cyclic steam production schemes, can oftentimes compromise the cement quality. Such operational scenarios are of particular concern in those cases where the initial quality of the cement column was marginal or poor, as further deterioration of the cement quality can potentially leading to undesirable gas flow paths in the wellbore.
Another factor that very often contributes to the type of surface casing vent flow and gas migration scenarios described above is the age of a well, as an overall deterioration of the wellbore can occur gradually over time. For example, production operations on the well may take place for extended periods of time, or perhaps even continuously for the entire life of the well, thus compounding the effects of the production and/or operational mechanisms described above. Furthermore, workover operations may have been performed on the well so as to open new pay zones within the reservoir, thus creating the potential for further zonal isolation problems. Moreover, the extended age of a well would generally serve to exacerbate any of the above-described marginal casing and/or cementing quality issues that might be present from the initial completion operations on the well. For example, some studies indicate that, of offshore wells located on the outer continental shelf in the Gulf of Mexico, there is a probability that 50% or more of wells that are at least 15 years or older have occurrences of uncontrolled gas flow, such as surface casing vent flow and/or gas migration.
The sources of uncontrolled gas flow from a wellbore can vary. For example, in some instances, surface casing vent flow and/or gas migration may be caused by gas moving through and/or from the producing formation, formations with commercial potential that are up-hole of the producing formation, or from non-commercial gas bearing zones or “stringers”. Furthermore, the flow characteristics of SCVF/GM occurrences can also fluctuate depending on a variety of well conditions, such as restriction points, surging, inter-zonal movement and charging, source depletion, and the presence of near wellbore and/or near formation flow paths.
However, irrespective of the source, many national or state/provincial regulations require that any such sources of uncontrolled gas flow be located and repaired, because in many oil and gas wells, the annulus between the surface casing of a well and the next smaller casing set inside of the surface casing therein must be left open to atmosphere. Accordingly, operators are often required to test the surface casing for a vent flow or gas migration, and effect repairs in accordance with applicable directives and regulations. Moreover, each of the various flow characteristics noted above (e.g., surging, inter-zonal movement, etc.) present different challenges in determining the lowest possible source within the wellbore that may be targeted for SCVF/GM repairs.
With the variety of conditions associated with uncontrolled gas flow around a wellbore, and the many different flow paths for gas to surface, the identification, repair and remediation of SCVF/GM issues can be a highly challenging process. Of paramount importance is the initial step of SCVF/GM source identification—in other words, properly pinpointing the location from which the gas leak and/or migration originates. Absent a reasonably conclusive identification of the gas source, any subsequent repair and/or remediation steps may only be partially successful, or in some situations may even be completely futile, depending on the overall characteristics of the wellbore.
One approach that is commonly used for detecting the location of SCVF/GM gas sources is to run a noise/temperature log on a well, which is often performed in conjunction with other well logging operations, such as a cement bond log and/or a gas identification neutron log and the like. Generally, noise/temperature logs are acquired during a typical well logging operation, and are commonly run by obtaining temperature data while running into a wellbore (i.e., down the hole), and obtaining noise samples while running back out of the wellbore (i.e., up the hole). During noise sample acquisition, the samples are recorded through a series of preselected, regularly spaced station stops using a pre-set recording interval at each station stop. Recording depth typically starts at the plugged back total depth (PBTD) of the well, and continues progressively to each regularly spaced station stop and up to surface. The noise logs are then analyzed to find occurrences of the type of audible characteristics and/or noise spectra that is most often associated with such undesirable gas flow events.
FIG. 1 schematically illustrates an exemplary prior art noise logging tool arrangement that has been used in an effort to detect and isolate sources of uncontrolled gas flow that may lead to surface casing vent flow and/or gas migration in a wellbore. FIG. 1 depicts a wellbore 150 that includes a casing 101, such as a production casing and the like, that is set in a formation 100 using a cement sheath 103. During a typical noise logging survey, a noise logging tool 110 is lowered down the bore 102 of the production casing 101 using a wireline 111, which is used to transmit noise logging data from the noise logging tool 110 to a wireline service vehicle (not shown) positioned at the surface near the wellhead of the wellbore 150. The wireline 111 is also used to support the noise logging tool 110 during the logging operation, and to move the noise logging tool 110 up and down the bore 102 of the production casing 101 between each of the pre-selected station stops described above.
As shown in FIG. 1, the noise logging tool 110 is positioned at a representative station stop within a zone of interest, or targeted noise logging zone 140, where a relevant gas source 120 that may cause an uncontrolled flow of gas 121 may be located. In some cases, the zone of interest 140 over which the noise logging data is acquired will extend above and below the noise logging tool 110 as shown in FIG. 1. The overall height of the zone of interest—i.e., the height corresponding to the distance between adjacent station stops—that may range on the order of 1-10 meters, although the specific overall height of the targeted noise logging zone 140 may also be larger or smaller, depending on various logging parameters, such as the type and design of the noise logging tool 110, the configuration of the wellbore 150 (e.g., the number and/or size of casings present in the zone 140), the type of strata in the formation 100, and the like. For example, in one well-known industry standard, the distance between station stops is approximately 5 meters, wherein 30-second sound samples are obtained at each station stop, however smaller station-stop distances may be chosen according to the specific well and/or flow conditions. During the pre-set recording interval, any noise that is created by the gas source 120, indicated in FIG. 1 as radiating sound waves 122, is detected by the noise logging tool 110, and the associated noise data is transmitted to the wireline service vehicle at the surface via the wireline 111.
There are, however, some limitations on the use of currently available noise logging technology for SCVF/GM source isolation, which can sometimes inhibit efforts to accurately pinpoint the gas source locations. For example, extraneous noises that are unrelated to the SCVF/GM source noise (generally represented in FIG. 1 as sound waves 130a traveling down the wellbore 150 and sound waves 130b traveling up the wellbore 150) sometimes occur during the noise logging operation. Such extraneous noises 130a, 130b are sometimes referred to as “rogue noise events,” and can often interfere with and/or affect the noise samples obtained by the noise logging tool 110 in the targeted noise logging zone 140 during the logging operation. Rogue noise events can include, among other things: noises generated by downhole tools; noises caused by tool movement; noises caused by debris inside the wellbore hitting a tool; and restrictive processes in the cement sheath 103 that can generate loud audible responses, such as formation sloughing in creating smaller flow channels, worm-hole channeling in the cement sheath 103, and the like. Such situations may lead to changes in the volumetric flow of gas behind the casing 101, e.g., by changing the flow diameter, changing the amount of gas flow (as with additional flow sources and the like), increasing the flow profile, etc.
The interpretation of the noise data obtained using the noise logging tool 110 at each depth of acquisition is generally adversely affected by any such rogue noise events that may exist during noise data acquisition. Since the noise data acquired by the noise logging tool 110 may contain the noise from an important SCVF/GM gas source related noise (such as that represented by the sound waves 122 from gas source 120 shown in FIG. 1) as well as all other detectable rogue noise events (such as those represented by the sound waves 130a, 130b), interpretation of the noise logging data may be difficult, and can often be incorrect. Furthermore, in some cases, rogue noise events can appear to have the same audible characteristics and/or noise spectra of gas movement within the wellbore 150, the cement sheath 103, or the adjacent formation. Moreover, irrespective of the noise source, it should be appreciated that sound waves readily propagate up and down the wellbore 150 via the various media and materials in and around the wellbore 150, such as any fluid in the wellbore 150, the well casing 101, the cement sheath 103, and the adjacent formation 100, wherein the majority of such noise propagation is believed to occur in the wellbore fluid and/or the well casing. Due to this sound wave propagation, the rogue noise events 130a, 130b could initiate from outside of the targeted noise logging zone 140. Furthermore, the true source and location of the noise can be masked to some degree, thus making it difficult to accurately assess whether the noise source is originating within the targeted noise logging zone 140 being logged, or in some other source interval above or below the tested zone 140, thus potentially leading to an improper interpretation of the noise data, and an erroneous identification of a relevant SCVF/GM source.
In other instances, the noise logging tool 110 can detect problematic noise events (e.g., the sound waves 130a, 130b) that are directly or indirectly related to other SCVF/GM sources around the wellbore 150 which are located outside of the targeted noise logging zone 140 that is currently being logged. For example, in some cases, the noise logging tool 110 may detect a noise that is generated by an SCVF/GM source that is either above or below the targeted noise logging zone 140 being tested, but which propagates up or down the wellbore 150 as described above. Such SCVF/GM sources located outside of the targeted noise logging zone 140 can also influence the noise data being acquired, thus making it difficult to accurately determine whether or not a relevant SCVF/GM source is present somewhere within the zone 140 that is being logged, because, even though a given wellbore noise may be related to the actual flow of problem gas, the sound associated with the release of gas from the formation into an annular space between the casing and the formation annulus may be masked by such rogue noise events.
In other cases, a rogue noise that is detected by the noise logging tool 110 may be indirectly associated with a relevant SCVF/GM source. For example, the noise logging tool 110 may detect noise that is caused by a flow of uncontrolled gas, such as the flow 121, after the gas has left the source 120 and as the flow 121 moves up the wellbore 150 through an annulus outside of the casing 101, a flow channel in the cement sheath 103, and/or around any other flow restrictions that may be present within or adjacent to the wellbore 150. In some instances, this could include charge and release events associated with a charging gas void in the cement sheath 103, which may release only under specific pressure conditions. Since such noise occurrences are also caused by a flow of gas in and around the wellbore 150, they can sometimes mask the true location of any uncontrolled gas source coming from the surrounding formation 100, as it can be very difficult to distinguish such noises from, for example, the noise 122 generated by a relevant gas source 120.
Accordingly, there is a need to develop and implement new designs for and methods of using downhole tools that facilitate the performance of noise logging operations, and which address and mitigate at least some of the problems that are associated with locating the sources of uncontrolled gas flows in and around oil and gas wellbores, as described above.